A standard definition of an output skew parameter OSLH for an IC clock driver circuit having a primary clock signal input and at least two secondary clock signal outputs is illustrated in FIG. 1. According to this definition, the single clock input signal provides a time zero reference t.sub.o for measuring propagation time tplh from the clock input signal at time t.sub.o to a specified point in the low to high (LH) transition at the selected output. Because of different characteristics of the multiple signal paths a range of different propagation times tplh for the secondary clock signals at the multiple outputs can be expected. The output skew parameter OSLH is the maximum difference in propagation times tplh between any two of the outputs of the multiple output clock driver circuit. Referring to FIG. 1, if the propagation time for the output signal on output 1 is the smallest propagation time tplh.sub.min, and the propagation time tplh for the output signal on output 2 is the greatest propagation time tplh.sub.max, then the output skew parameter OSLH for the device may be defined as: EQU OSLH=tplh.sub.max -tplh.sub.min.
Similarly the output skew parameter OSHL for high to low transitions HL is defined as EQU OSHL=tphl.sub.max -tphl.sub.min.
It is noted that the output skew parameters OSLH and OSHL represent the timing skew or timing difference between the multiple output edges themselves without any necessary reference to the time t.sub.o of the corresponding input signal edge. A conventional device for measuring the output skew parameters OSLH and OSHL is the MCT Tester, a microprocessor controlled digital IC tester described for example in the MCT 2000R TEST SYSTEMS HARDWARE MANUAL, Publication No. 010193B, Revision B, Nov. 1, 1986, .COPYRGT. 1986 MCT, Micro Component Technology Inc., 38 North Victoria Street, P.O. Box 64013, St. Paul, Minn. 55164. A disadvantage of the MCT Tester and other conventional testers is that output timing skew between multiple outputs signals cannot be measured directly. Rather, signal propagation times tplh and tphl are measured with reference to an input signal. Output skew is therefore a derivative parameter subject to the variables in measurement of separate propagation times through the part from input to multiple outputs.
A further problem with such conventional testers is the limited bandwidth and limited resolution available for testing. As the speed of IC parts and devices has increased, equalling and exceeding the speed of the tester, large errors in testing measurement occur. For example, the tester oscilloscope has a certain limited rise time as does the device under test. As the rise time of the DUT part approaches the rise time of the tester the result is increasing error in the rise time of output signals displayed on the screen. Skew testing for high speed low skew parts in particular eludes conventional testers.